In daily life, many people have a habit of using a smart device (such as a mobile phone) before falling asleep. If, at such a time, ambient lights are turned on, power may be wasted and other people may be affected. If ambient lights are not turned on, in order to use the mobile phone in an environment with extremely low brightness (such as at nighttime with an exemplary brightness of 0.001 lux), most people choose to decrease the brightness of the mobile phone. However, because the display brightness is constrained by the LCD backlight circuit, structure, control mode, and cost, it is difficult to achieve extremely low display brightness. The inability to achieve an extremely low display brightness may cause great damage to the eyes when using the mobile phone in a very dark environment (such as at nighttime with an exemplary brightness of 0.001 lux).